A Classic Case of Milk Fat Depression

I was reading a paper about milk fat depression published in the October 2013 Journal of Dairy Science. One of my client’s dairies had just experienced a case of milk fat depression. This rarely occurs, but, when it does, there’s no doubt what’s happening. The timeline on this dairy followed the experimentally induced milk fat depression in this study almost to a "T."

Milk fat depression occurs when all the stars align in the diet and hence, the rumen, even in well formulated diets. High fat (especially unsaturated fat), combined with high starch or low fiber alter the biohydrogenation pathways in the rumen, producing trans fatty acids that inhibit milk fat synthesis in the mammary gland. Rumensin seems to accentuate these changes.

According to the researchers, the timeline for milk fat depression follows dietary induced changes in rumen microbial populations. Unsaturated fatty acids in the ration cause changes in microbial pathways as soon as two to three days after ration changes. But full milk fat depression takes about 14-19 days as microbial shifts are complete. Milk fat can be restored by changing the diet (reducing oil, increasing forage) and the recovery follows a similar pattern, as rumen microbial populations adjust to the new diet.

In our case, butterfat was running about 3.7% prior to the ration changes. We were running low on alfalfa haylage due to a loss of first cutting. Because of this, we had to increase the proportion of corn silage in the diet. Then our whole cottonseed contract ran out, so tallow replaced the fat provided by the cottonseed. We were also feeding about 400 mg of Rumensin. Dietary fiber was similar before and after the changes, although the physically effective fiber had changed due to less haylage and no cottonseed.

Cows were ruminating well and manure looked normal, but butterfat started slipping a few days after cottonseed was replaced with tallow plus additional corn silage and protein. Butterfat was down to less than 3.0% within three weeks.

It was apparent that the combination of more corn silage, lower effective fiber and the tallow had caused a microbial shift that resulted in production of milk fat robbing fatty acids. Corn and corn silage can contribute a lot of unsaturated corn oil.

We brought in some hay, reduced the tallow and replaced it with a rumen inert bypass fat. We also bought some more cottonseed. Butterfat began turning in a few days, with a full recovery in about two weeks.

In retrospect, we should have replaced the whole cottonseed with a rumen inert fat rather than tallow. Even though we may know how to fix milk fat depression, the rumen microbial population is slow to adjust, so two to three weeks of low butterfat can be expected.